In each of the following, determine whether or not $H$ is a subgroup of $G$. (Assume that the operation of $H$ is the same as that of $G$.)
$ G=\langle\mathbb{R} \times \mathbb{R},+\rangle, H=\left\{(x, y): x^{2}+y^{2}>0\right\} . \quad H$ is $\square$ is not $\square$ a subgroup of $G$.